The invention concerns a process for reducing the oxygen content of the atmosphere in a heat treatment furnace for ceramic materials, preferably soft-magnetic ferrites, to a prescribed value and a furnace for carrying out this process.
Heat treatment furnaces for the heat treatment of ceramic materials are known in the form of continuous heat treatment furnaces or batch operating furnaces. If materials of conventional composition, such as porcelain and similar materials, are burnt in these furnaces, no problems worth mentioning occur in matching the composition of the furnace atmosphere to the chemical nature of the charge material. Many of these materials have to be heat treated in an oxidizing atmosphere; in this case, a definite quantity of air or oxygen is added to the furnace atmosphere. Other charge materials require neutral or reducing atmospheres and, in this case also, there is no difficulty in adjusting the furnace atmosphere correspondingly.
For use in the electronics field, in particular, increasing use is made of ferrites, that is ferro-spinels, which, in addition to Fe.sub.2 O.sub.3, also contain other metal oxides, for example zinc oxide, nickel oxide, manganese oxide and the like.
The soft-magnetic zinc/manganese ferrites, in particular, have the property of releasing oxygen in substantial quantities when heated and thus altering the composition of the furnace atmosphere. Since, on the other hand, the magnetic properties of the materials depend to a substantial extent on the composition of the heat treatment atmosphere, it is precisely this "outgassing" of oxygen from the heat treatment charge which provides substantial technical difficulties.
Similar heat treatment behavior is exhibited by bodies made of pure Fe.sub.2 O.sub.3, which, as is known, are converted at increased temperatures into Fe.sub.3 O.sub.4, the so-called magnetite, giving off oxygen in the process. In this case also, the furnace atmosphere is thus enriched in oxygen during the heat treatment process.
It would be fundamentally possible to dilute the outgassing oxygen from the heat treatment charge by an increased supply of nitrogen sufficiently large to obviate the damaging effects of the oxygen. The quantity of nitrogen necessary for this purpose, however, would be so large that it would critically disturb the temperature balance of the furnace so that additional measures would have to be taken in order to heat the necessary quantity of nitrogen to the temperature existing in the introduction zone before introducing it into the furnace. Furnaces equipped in this manner must, in consequence, be provided with additional burners and recuperative chambers and they demand a substantially increased fuel requirement.